North Korea Close to Having Nuclear-Armed ICBM: Official

North Korea now has an intercontinental ballistic missile, or ICBM, armed with a nuclear warhead capable of reaching the U.S. mainland, but they have yet to solve the "re-entry problem" that would burn up the warhead as it hits the atmosphere, a senior military official said Thursday.

"It's the threat that keeps me awake at night," the senior official said.

North Korea has acquired and developed an ICBM and perfected the miniaturization of the warhead to fit atop the missile, the official said, but "they’re not sure of the re-entry capability" as the missile transitions from space to the atmosphere.

In recognition of the growing North Korea threat, the Defense Department has transferred some authority for countering weapons of mass destruction from U.S. Strategic Command to the Joint Special Operations Command, according to the high-ranking official who spoke on grounds of anonymity.

Earlier this year, the Defense Department confirmed that North Korea had developed a new mobile ICBM capable of hitting the U.S. called the KN-14. The new missile was on display in a military parade last year in October in Pyongyang, the North Korean capital.

North Korea also has the long-range Taepodong-2 missile, a two or three-stage missile that has successfully delivered satellites into space.

In 2011, then-Defense Secretary Robert Gates warned that "North Korea will have developed" an ICBM by 2016, but said the missile would have limited capabilities. Since then, North Korea has aggressively pursued development of a miniaturized warhead, according to the Arms Control Association.

In January, North Korea conducted its fourth underground nuclear test and leader Kim Jong-un boasted that it was a hydrogen bomb, though U.S. analysts dismissed the claim. Two months later, North Korea unveiled a mock-up of a miniaturized nuclear bomb and performed two separate missile-related ground tests, the Arms Control Association said.

Last year, Army Gen. Curtis M. "Mike" Scaparrotti testified before the Senate Armed Services Committee on his concerns about North Korea's progress on fitting a warhead to an ICBM.

"I believe they've had the time and the capability to miniaturize" a warhead, said Scaparrotti, then the commander of U.S. Forces-Korea and now the supreme allied commander of NATO.

Last week, a former U.S. commander in South Korea  said the new administration of President-elect Donald Trump will have to consider a pre-emptive strike on North Korea before it can launch a long-range missile.

"I don’t think any talking, any diplomacy, is going to convince Kim Jong-un to change," retired Army Gen. Walter Sharp said of the North Korean leader in suggesting the possibility of a pre-emptive strike to eliminate the nuclear threat.

Should North Korea put a missile such as the three-stage Taepodong 2 on the launchpad, and the U.S. was unsure whether it carried a satellite or a nuclear warhead, the missile should be destroyed, said Sharp, the former commander of U.S. Forces-Korea and the United Nations Command from 2008 to 2011.